A honeycomb structure including a plurality of cells partitioned by a plurality of partition walls which are made of porous ceramics and are lined up in a single direction is conventionally used as a filter which traps and removes particulate matters (hereinafter, referred to as “PM”) such as soot contained in the gas exhausted from a diesel engine. In such a honeycomb structure, generally, cells one ends of which are sealed and cells the other ends of which are sealed are alternatively arranged. The gas flows in from the cells opened in one direction, passes through the porous partition walls, and flows out from the cells opened in the other direction. Then, when the gas passes through the partition walls, PMs in the gas are trapped and removed through surfaces and pores of the porous partition walls.
Therefore, when pore diameters are too large, PMs which pass through the partition walls without being trapped are increased so that filtration efficiency is lowered. On the other hand, when pore diameters are too small, pressure loss becomes large due to resistance to passage of the gas so that the load on the engine is increased. Then, in order to balance filtration efficiency and pressure loss which are inversely correlated, there is proposed a filter in which pore diameters of porous ceramics constituting the partition walls are controlled (see, for example, Patent Documents 1 and 2).
These filters are intended to have relative number of pores suitable for the trapping of PMs by setting pore diameters of the partition walls in a limited narrow range. An average value of the pore diameters measured by mercury porosimetry is set to be 1 μm to 15 μm in the former (Patent Document 1), and 20 μm to 60 μm in the latter (Patent Document 2), a standard deviation in a pore diameter distribution is set to be 0.20 or less when the pore diameter is represented by common logarithm.
[Patent Document 1] Japanese Patent No. 3272746
[Patent Document 2] Japanese Patent Application Laid-open No. 2002-242655